One Laser, Six Entangled Modes

Stewart Wills

An OPO consists of a nonlinear optical crystal within an optical cavity. When pumped by an external laser of the right frequency, the crystal down-converts the pump beam into two beams of lower frequency, commonly called the “signal” and “idler” beams. The nonlinear crystal also acts as a gain medium, through the process of parametric amplification, giving rise to oscillation and to powerful signal and idler output beams once the pump power crosses a specific threshold.
A research team in Brazil has used an OPO to demonstrate the simultaneous entanglement of six optical modes in one cavity, using a single continuous-wave (CW) laser source (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.121.073601; Phys. Rev. A, doi: 10.1103/PhysRevA.98.023823). What’s more, the researchers showed that they could control the system’s level of entanglement via a single, easily accessed parameter—the power of the pump laser. 
The team began with a monochromatic (green) pump field from a doubled, filtered Nd:YAG laser, tied to the OPO using a partly reflective input coupler. Within the OPO, the light enters a nonlinear potassium titanyl phosphate crystal, where part of the pump light is downconverted into infrared signal and idler beams. Next, the team jacked up the pump power to 75 percent above the OPO’s oscillation threshold, and cooled the crystal down to 260 K to limit thermal noise.
The team found that the covariance matrix measurements agreed well with the values expected from numerical calculations for a six-part entangled system. And the degree of sideband entanglement could be improved by boosting the pump power. Above a certain power level, however, the beams become coupled with phonons in thermal reservoirs in the crystal, which introduces noise and degrades the level of entanglement. That problem, according to the team, should be surmountable by further cooling down the nonlinear crystal.

Setup for reconstructing the covariance matrix from the pump, signal and idler beams of an OPO. PBS, polarizing beam splitter; BS, 50:50 beam splitter; HS, harmonic separator; KTP, potassium titanyl phosphate nonlinear crystal; IC, input coupler; OC, output coupler (OPO cavity); FR, Faraday rotator. [Image: Reprinted figure with permission from F.A.S. Barbosa et al., Phys. Rev. Lett. 121, 073601 (2018), doi: 10.1103/PhysRevLett.121.073601; copyright 2018 by the American Physical Society]
The resulting amplified signal and idler beams, as well as reflected, non-down-converted pump light, were collected in three separate “analysis cavities” to allow for fine-tuning, and then traveled on to three pairs of photodetectors. The photocurrent output from the photodetectors allowed the in-phase and quadrature quantum-optical components of the field to be measured. Finally, the team plugged in those measured values to create a covariance matrix for the system, and used the eigenvalues of that matrix to determine the entanglement state of the six sidebands of the pump, signal and idler fields.
The entanglement of six optical modes from a single CW laser effectively doubles the three-part entanglement achieved in earlier OPO experiments. In principle, of course, the team’s experiments also show that no more than six optical modes can be entangled in an OPO system such as this—there are, after all, only three beams and six sidebands to work with. But the Brazilian group believes that putting together multiple OPO systems and lasers could enable them to entangle larger numbers of optical modes.
Publish Date: 10 September 2018 by OSA